A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol or natural gas.
Typical examples of fuel cells are polymer electrolyte membrane fuel cells (PEMFC) and direct oxidation fuel cells (DOFC). A direct oxidation fuel cell which uses methanol as a fuel is called a direct methanol fuel cell (DMFC). The polymer electrolyte membrane fuel cell is an environmentally-friendly energy source that can replace fossil fuel energy. It has advantages such as high power output density, high energy conversion efficiency, operability at room temperature, and the capability to be down-sized and tightly sealed. Therefore, it can be widely applied to various areas such as non-polluting automobiles, residential electricity generation systems, and as portable power sources for mobile communication equipment and military equipment.
The polymer electrolyte membrane fuel cell has the advantage of having high energy density, but also has the problems of requiring careful handling of the hydrogen gas, or requiring accessory facilities such as a fuel reforming processor for reforming fuel gas such as methane, methanol, or natural gas to produce the hydrogen required.
In contrast, a direct oxidation fuel cell generally has a lower energy density than a polymer electrolyte fuel cell, but has the advantage of easy handling of the liquid-type fuel, low operation temperatures, and does not require additional fuel reforming processors. Therefore, such direct oxidation fuel cells may be appropriate systems for small-scale and general purpose portable power sources.
In a fuel cell, the stack that actually generates electricity includes several to scores of unit cells stacked in multi-layers, each unit cell made up of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (referred to as a fuel electrode or an oxidation electrode) and a cathode (referred to as an air electrode or a reduction electrode) separated from one another by an electrolyte membrane.
A perfluorosulfonic acid resin membrane (NAFION®) having good conductivity, mechanical properties and chemical resistance has commonly been used for the polymer electrolyte membrane.
In general, a thicker perfluorosulfonic acid resin membrane provides better dimensional stability and mechanical properties, but increased membrane resistance. A thinner membrane provides lower membrane resistance, but worse mechanical properties whereby unreacted fuel gas and liquid tend to pass through the polymer membrane resulting in lost unreacted fuel during operation and lower performance of the cell.
Particularly, a polymer electrolyte membrane thermally compressed with a platinum catalyst electrode undergoes a change of 15 to 30% in membrane thickness and volume depending on temperature and degree of hydration, and results in a volume change of over 200% maximum with a 3 to 50 wt % methanol fuel. The thickness increases by such swelling of the electrolyte membrane and applies stress to the gas diffusion layer which is the electrode substrate. A change in the surface dimensions of the membrane induces physical deterioration of the interface between the catalyst particles and electrolyte membrane when the fuel cell is operated for long durations.